Hessian Fly on Wheat

John C. French Sr., Retired, Universities:Auburn, GA, Clemson and U of MO, Bugwood.org

During the past decade, damage to wheat by Hessian fly has been minimal in Pennsylvania. Outbreaks of this historic pest, however, have occurred in states to our south such as North Carolina and Virginia, possibly because growers have gotten lax about planting after the fly-free dates and/or increased use of small grain cover crops that are planted earlier in the year than a cash crop would be and then are not harvested.

The low incidence of occurrence of and damage from Hessian fly in Pennsylvania can be attributed both host-plant resistance and planting date. Many varieties of wheat commonly used in Pennsylvania possess either complete or moderate resistance to attack by Hessian fly. Also helpful in holding down fly damage has been the cultural practice of planting wheat after the "fly-free" date in late September or early October (see below for these dates). Continued diligence in using resistant varieties and planting late should prevent Hessian fly from returning as a major pest of wheat in Pennsylvania.

Life History

Hessian fly eggs. #512-11. Oregon State University in cooperation with EPA.

Hessian fly pupa. #512-25. Oregon State University in cooperation with EPA.

There are two generations per year, one in early fall and one in the spring. There may also be a third generation in July and August if moisture conditions are high enough for volunteer wheat to sprout and grow. When a summer or supplementary brood does develop, early-sown non-resistant wheat is often severely damaged.

Flies (Fig. 1) of the fall brood tend to appear in late September and live for just a few days. They lay their eggs on the leaves of young grasses, including small grains. Wheat appears to be preferred, but flies will also lay eggs on barley and rye and some native grasses. Planting after the fly-free date is an effective control method because it avoids having large numbers of young potential host plants available to receive eggs when adult flies are active. If the flies emerge and die off before the new wheat plants emerge, the crop cannot be infested. Maggots can survive on native grasses, but populations do not appear to thrive until they find small grain hosts, particularly wheat.

Fig. 1. Hessian fly adult. Photo from the USDA.

Maggots soon hatch from the eggs, and these tiny larvae crawl to the crown of seedlings (just above the roots) and feed on plant juices after injecting their unique saliva. Feeding by one larva can permanently stunt plant growth (Fig. 2). A larva will complete its growth before cold weather and pass the winter as puparium or "flaxseed."

Fig. 2. A hessian-fly infested wheat plant on left compared to a health plant on the right. Photo by John Tooker

Damage

In the spring, adult flies emerge from the "flaxseeds" and lay eggs on the leaves. Upon hatching, the maggots work their way under the leaf sheath near the node of these large plants. Their feeding at this site weakens the stem which results in the stalks breaking over before harvest. The maggots change into puparia about the time wheat heads out and they remain in the stubble as "flaxseeds" until fall.

Control

High mortality of Hessian fly from parasites is sporadic in the spring generations but cannot be relied upon for the high level of control we desire. Therefore we suggest the use of fly resistant varieties where feasible or plant wheat after the "fly-free date". The fly-free date for each county in Pennsylvania is listed below. It must be acknowledged, however, that these dates may be somewhat 'soft' given the warmer temperatures that we have been experiencing in recent years. To be more conservative, growers should consider planting even a week or so later if possible.

County

Fly-free Date

Adams

October 1

Allegheny

September 28

Armstrong

September 28

Beaver

September 28

Bedford

October 1

Berks

September 27

Blair

October 1

Bradford

September 26

Bucks

September 30

Butler

September 28

Cambria

September 27

Cameron

September 27

Carbon

September 27

Centre

September 27

Chester

September 30

Clarion

September 28

Clearfield

September 27

Clinton

September 27

Columbia

September 27

Crawford

September 26

Cumberland

October 1

Dauphin

September 27

Delaware

September 30

Elk

September 27

Erie

September 26

Fayette

September 28

Forest

September 26

Franklin

October 1

Fulton

October 1

Greene

October 1

Huntingdon

October 1

Indiana

September 28

Jefferson

September 28

Juniata

October 1

Lackawanna

September 26

Lancaster

September 30

Lawrence

September 28

Lebanon

September 27

Lehigh

September 27

Luzerne

September 27

Lycoming

September 27

McKean

September 22

Mercer

September 26

Mifflin

October 1

Monroe

September 27

Montgomery

September 30

Montour

September 27

Northampton

September 27

Northumberland

September 27

Perry

October 1

Philadelphia

September 30

Pike

September 26

Potter

September 20

Schulykill

September 27

Snyder

September 27

Somerset

September 27

Sullivan

September 25

Susquehanna

September 27

Tioga

September 26

Union

September 27

Venango

September 26

Warren

September 26

Washington

October 1

Wayne

September 26

Westmoreland

September 28

Wyoming

September 26

York

October 1

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: John Tooker, Assistant Professor of Entomology

Last updated: August 2012

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